Re: Serious performance regression in diff between 1.6.6 and 1.7.0

From: Brian Downing <bdowning@lavos.net>
Date: Jun 10, 2010, 17:08 UTC
Message-ID: <20100610170804.GB2635@glaurung.lavos.net>
In-Reply-To: <20100610001005.GA2635@glaurung.lavos.net>

On Wed, Jun 09, 2010 at 07:10:05PM -0500, Brian Downing wrote:

> I've stumbled across a pretty serious performance regression in "git
> diff" for a large (187456 files checked out on HEAD) repository of mine.

[...]

Show 7 quoted lines

> It is more than 10x worse for my case here:
> 
> before 730f72840cc50c523fe4cdd796ea2d2fc4571a28:
> 0.30user 0.50system 0:00.81elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
> 
> 730f72840cc50c523fe4cdd796ea2d2fc4571a28:
> 5.58user 0.40system 0:05.98elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k

Here's a little more profiling information (from the bad case). I ran this a couple of times to see what oprofile said. Sorry about the lack of non-Git symbols:

CPU: Core 2, speed 2668 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100000
samples  %        image name               app name                 symbol name
499255   62.8497  git-diff                 git-diff                 df_name_compare
110561   13.9182  git-diff                 git-diff                 do_compare_entry
54508     6.8619  no-vmlinux               git-diff                 (no symbols)
33741     4.2476  no-vmlinux               no-vmlinux               (no symbols)
23724     2.9865  git-diff                 git-diff                 ce_in_traverse_path
11767     1.4813  libcrypto.so.0.9.8       git-diff                 (no symbols)
10511     1.3232  git-diff                 git-diff                 find_cache_pos

I also ran this through callgrind to see how often the above were called:

        Calls  Symbol
  -----------  -------------------
      197,958  unpack_callback
      208,460  find_cache_pos
   37,308,336  ce_in_traverse_path
  156,950,469  do_compare_entry
  156,950,469  df_name_compare

Hope this helps.

-bcd